1. Field of the Invention
The present invention relates to a method for cooling a steel strip in a continuous-annealing furnace wherein, to cool the steel strip, the steel strip is brought into contact with a cooling roll having a structure which allows the passage of a cooling medium therethrough.
2. Description of the Prior Art
It is known to wind a steel strip, at a certain winding angle, on a roll(s) having a hollow aperture, the roll(s) being disposed in a continuous-annealing furnace, and to flow a cooling medium through the hollow aperture so as to cool the steel strip (c.f., for example, Nippon Kokan Technical Report No. 96, 1982, "Application of a Water-Cooled-Roll Technique to an NKK CAL Process"). This type of cooling involves an essentially unstable characteristic. That is, upon the generation of an unstable cooling state, the unstable cooling state is magnified. More specifically, if, with respect to one roll, superfluously one part of the steel strip is cooled compared to the other parts as seen in the traversal direction of the strip, thermal shrinkage of the one part occurs, and, hence, a greater tensional force is induced in the one part than in the other parts, which results in an increase in the contact pressure between the steel strip and the one roll or a succeeding roll and hence an increase in the heat transfer quantity. Thus, the phenomenon of cooling of the a superfluously cooled part of the steel strip is successively magnified or amplified. As a result, frequently the qualities of the product are nonuniform as seen in the short width direction of the steel strip and a serious shape failure which sometimes accompanies bending may occur. As one measure for preventing shape failure, the tensional force imparted to a steel strip being conveyed is enhanced so as to provide a uniform contact between the steel strip and the cooling roll. However, since the yield point of the steel strip is low at the high-temperature side of the cooling-temperature range, the tensional force imparted is restricted so as not to exceed the yield point, and, therefore, this measure cannot completely solve the above-mentioned problems. As is described above, the mechanism of roll-cooling is essentially unstable. A stable mechanism of roll-cooling is only attained by the provision of means for controlling the roll-cooling quantity as seen in the short width direction of a steel strip.
Various methods for controlling the roll-cooling quantity as seen in the short width direction of a steel strip have been disclosed. For example, Japanese Examined Patent Publication No. 57-49097 dicloses a controlling method in which the cooling-medium channel in the cooling roll is separated into a plurality of channels and the flow rate of the cooling medium in each channel is controlled as seen in the short width direction of a steel strip. However, satisfactory cooling cannot be expected in this disclosed method since the heat flow rate from the steel strip to the cooling roll is predominantly determined by the contact heat conductance at the contact portion of a steel strip and the cooling roll. Hence, the heat resistance in the cooling-medium channel is generally small.
According to another controlling method, i.e., the one disclosed in Japanese Unexamined Patent Publication No. 57-116734, the cooling-medium channel is separated into a plurality of channels as seen in the short width direction, and the pressure of the cooling medium in each channel is varied to change the roll crown of the cooling roll. In this method, a high pressure is necessary, thereby making the investment cost enormous.
According to still another controlling method disclosed in Japanese Unexamined Patent Publication No. 56-41321, a gas jet is blown from behind the cooling roll onto the edge portions of a steel strip, at which edge portions contact failure between the cooling roll and the steel strip is likely to occur, the edge portions additionally being cooled by the gas jet. However, since a portion of a steel strip where nonuniform contact between the cooling roll and the steel strip occurs is not limited to the edge portions, the disclosed method cannot attain a satisfactorily uniform cooling.
According to yet another controlling method disclosed in Japanese Examined Patent Publication No. 56-10973, a plurality of gas-jet nozzles are disposed adjacent to the rear surface of the cooling roll in an attempt to make the cooling more uniform. The utility of this method, however, is poor because once a great nonuniformity in the tensional force distribution is generated in a steel strip which is wound around the cooling roll, an extremely strong gas jet is necessary to correct the tensional force distribution.